1. Field of the Invention
The present invention relates to image processing, and particularly, to an object image detection device and method able to detect an inclined object image from an input image.
2. Description of the Related Art
In recent years, usage of an object image detection technique has been widely spread. Here, the “object image detection technique” represents a technique of detecting objects such as faces or cars, from an image such as a picture, a still image, or a video image. Particularly, a face detection technique is widely used in digital cameras, security, and other fields for detecting positions, size, and other information of features of the faces. For example, considering a digital camera, the detection results are used for focusing, exposure, white color balance, or other adjustments of the faces.
When using a digital camera, depending on situations, a user may incline the digital camera and take pictures having different orientations relative to the monitor of the digital camera compared to a normal condition in which the monitor of the digital camera is not inclined but held in a horizontal direction. In this case, pictures inclined relative to the monitor of the digital camera are obtained.
FIG. 1A through FIG. 1C illustrate different orientations of the digital camera.
In FIG. 1A, the digital camera is held in the normal condition, namely, the digital camera is not inclined but in the horizontal direction.
In FIG. 1B, the digital camera is rotated by 90° counter-clockwise relative to the horizontal direction.
In FIG. 1C, the digital camera is rotated by 90° clockwise relative to the horizontal direction.
On the other hand, in image processing of an object image detection device installed in the digital camera, the origin of a coordinate system is fixed at a specified position of the monitor, for example, at the left-upper corner of the monitor. For this reason, when the digital camera is inclined to take pictures, the object image detection device receives face image signals having a different orientation from the image having the normal orientation.
FIG. 2A through FIG. 2C illustrate face images having different orientations.
The face images illustrated in FIG. 2A through FIG. 2C are taken with the digital camera having orientations as shown in FIG. 1A through FIG. 1C, respectively.
For example, the technique of detecting face images having different orientations is disclosed in U.S. patent application Ser. No. 11/299,504 (hereinafter, referred to as “reference 1”), and U.S. patent application Ser. No. 11/285,172 (hereinafter, referred to as “reference 2”).
A general technique of detecting face images is disclosed in “Robust Real-Time Face Detection”, P. Viola, M. J. Jones, International Journal of Computer Vision, vol. 57, pp. 137-154, May 2004 (hereinafter, referred to as “reference 3”).
However, the technique of detecting face images disclosed in reference 1 and reference 2 requires a great deal of computation. On the other hand, portable digital devices like digital cameras have processors of low clock frequencies and memories of small capacities, thus lack sufficient computational capability. As a result, the face image detection technique in the related art cannot be applied directly.